This invention relates to the production of maleic anhydride and in particular to an improved process and apparatus pertinent to the stage of the dehydration of maleic acid.
The synthesis of maleic anhydride by the oxidation of benzene or C.sub.4 hydrocarbons (butane, butenes, butadiene, or C.sub.4 cuts resulting from the vapor-phase crakcing of petroleum cuts) leads to a product containing deleterious impurities. One of the best known methods of eliminating these impurities involves the washing of the oxidation product with water in order to convert it at least partially into maleic acid and then subjecting the resultant solution to dehydration to form maleic anhydride.
For the dehydration step, French Pat. No. 2,062,821 describes the dehydration of maleic acid into maleic anhydride in two stages: (A) concentrating a solution of maleic acid having a content from 30 to 50% by weight up to more than 90% in at least one agitated thin-film evaporator; and (B) dehydrating the concentrated solution of maleic acid into maleic anhydride while simultaneously eliminating the impurities, the main impurities being fumaric acid. Likewise, in French Pat. Nos. 1,524,319 and 2,242,388, countercurrent agitated thin-film evaporators are employed. The reason why these particular evaporators are employed is high heat transfer coefficient, short residence time, liquid film wiping and limitation of liquid entrainment by the effect of the reactor.
Conversely, the employment of these countercurrent agitated thin-film evaporators causes difficulties becasue of their mechanical complexity. The moving rotor results in vibrations, requires additional energy consumption and necessitates the employment of lubricating oil and cooling liquid for the bearings. Also, a simple motor failure or seizing of the rotor leads to stoppage of the whole plant. Finally, owing to mechanical restraints of these apparatus, they are limited in evaporation area and in the magnitude of the heat exchange coefficient, consequently limiting the productive capacity of the plant.
In a different evaporation technique, British Pat. No. 737,391 describes the concentration and dehydration of a solution of maleic acid in a battery of three single-tube horizontal evaporators which operate cocurrently and are mounted in series.